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Possible Duplicate:
How does IPv4 Subnetting Work?

I have a list of IP ranges I would like to ban, an example being:


Where xx is anything.

I've added the domain and IP restrictions into IIS. When I click add deny entry, I see:

enter image description here

For my above example, what should I enter as the values?

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marked as duplicate by Chris S Oct 7 '12 at 17:11

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

up vote 12 down vote accepted

IP Address Range:
Mask or Prefix:

Other examples:

Ban the lower half: -

IP Address Range:
Mask or Prefix:

Ban the upper half: -

IP Address Range:
Mask or Prefix:

In last two examples, the mask is also known as a "/25", because 25 of the first 32 bits of the address are part of the network address, and the remaining 7 bits are used for host addresses.

Probably a good idea to read up on subnetting, if you need to have a thorough understanding.

Or use an online calculator. This one is fairly decent:

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The mask/prefix confuses me, should it always be – Tom Gullen Oct 7 '12 at 16:06
No, it would depend on the scope of addresses that you wanted to ban. I will insert a few more examples. – Greg Askew Oct 7 '12 at 16:34

IP address range:

Mask or Prefix:

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